10 years ago
It depends on your take on things and what you class as 'cannon'
Prometheus is rumoured to finally explain to us that the Aliens were created as bio-weapons by the Space Jockeys/engineers as a means to 'drop and cleanse' planets - or in the new films' case - as a punishment to humans for trying to steal the secrets of creation for their own personal gain.
Some choose to believe the xenomorphs evolved on their own, others believe the Jockeys created them to aid in the war on the predators.
It's all presonal preference, but in my opinion, i'll stick to the theories from the films (excluding AVP, of course)